Exhaust lines are known that include a reduction system for nitrogen oxides and a device for injecting a reducing agent, such as urea, placed upstream from that system. The most widespread configuration of the urea injection areas is generally situated between a particle filter (preceded by an oxidation catalyst) upstream from a selective catalytic reducer (SCR) of the nitrogen oxides downstream. Another relatively widespread solution consists of placing the injection area between the oxidation catalyst and either a particle filter that is impregnated to treat the reduction of the nitrogen oxides, or an SCR catalyst followed by a traditional particle filter.
However, in both cases, the current injection areas comprise, from the outlet face of the upstream monolith (which is either a particle filter or an oxidation catalyst) as far as the inlet face of the following monolith (which is either an SCR catalyst, or an SCR impregnated particle filter, or an SCR catalyst followed by a standard particle filter): a convergent cone that reduces the passage diameter of the gases, a tube supporting the injector, and a divergent inlet cone that increases the passage diameter of the gases. Furthermore, in most cases, the injection areas comprise a mixer inside the tube after the support for the injector.
Such an arrangement requires an incompressible length in particular due to the presence of the convergent and divergent cones. Furthermore, to ensure correct operation of the urea injection system, it is necessary for the injection, evaporation, hydrolysis-thermolysis of the urea into ammonia, and mixing of the ammonia with exhaust gases functions to be performed in order to obtain a very homogenous dispersion of the ammonia in the exhaust gases on the inlet face of the downstream monolith. This conversion of the urea into ammonia and mixing between the ammonia and the exhaust gases requires time, and consequently a significant travel distance.
Overall, optimizing the various functions as much as possible, the distance between the two monoliths may be reduced to 200 mm. However, such an arrangement with a reduced distance is delicate and costly to produce.
Certain exhaust lines are equipped with devices for injecting ammonia in gaseous form, replacing the injection of urea. The portion for injecting gaseous ammonia can be situated between the two monoliths, which also requires an incompressible length between the monoliths. In any case, this length must allow good mixing between the injected ammonia and the exhaust gases.